Checking for an existing word in a file
Your code looks quite neat, although note that formatting, including spaces
around operators etc, line lengths, defining multiple variable per line, should follow a conventional style.  

There are some issues, the main one being that compiling and running the code,
it does not work.  The problem is that

fopen(filename, "a+");


opens the file for  reading and writing but leaves the stream pointer at the
end of the file.  So any subsequent read will fail.   You need a rewind();
or equivalent (fseek etc) after checking that the file opened successfully.

After fixing the rewind, another issue is that fgets returns the input word
with a trailing newline, \n.  Your code ignores this and manages to work
because the strings that are written to and read from the dictionary also have
a trailing \n.  But this fails when a user enters a string that fills your
buffers, 19 characters or more.  In this case, the fgets call cannot read
the whole string and reads just the 1st 19 chars (and adds a terminating \0),
leaving the remaining chars in the input stream.  In this case there is no
trailing \n in the buffer.  The next time you read from stdin the 20th and 21st chars are
read and treated as the y/n answer.  Even if these chars happen to be "y", the
19-char word is written to the dictionary without a trailing \n, corrupting
the dictionary (try looking up abcdefghijklmnopqrsy - notice the 'y' at the end)

There are ways to fix these problems, for example you could try using
getline() to get the input string instead of fgets, although getline is
more tricky to use correctly.   Alternatively (as it is just an exercise)
just check the input string to confirm that there is a trailing \n (meaning
that the whole line was read) and reject the word and fflush the input if not.   To do things
properly you really need to remove the trailing \n from the user input string and
from the string read from the dictionary before comparing, and add \n back when writing to the
file.

Another issue is poor comments.  Comments must be useful; otherwise they are
just noise (and hence make reading the code more difficult than it need be).
All of your comments are just noise.

Note also that when opening the dictionary fails you should exit. There are numerous other issues, but I'll leave it at that for now...

EDIT

As no reviewers have commented on other issues, here are some other things I
spotted:


avoid use of constants such as 20.  Instead, define the buffer size with a
define and use sizeof where you need the buffer size. For example:

#define BUF_SIZE 20
...
char buf[BUF_SIZE];
fgets(buf, sizeof buf, stdin);


by using sizeof you guarantee that the size is correct - but note that
if you use it on a pointer, then you get the wrong value:

char *buf = malloc(BUF_SIZE);
fgets(buf, sizeof buf, stdin); // WRONG!!


in the latter case, sizeof returns the size of the pointer (typically 4
or 8 bytes) - you need to pass BUF_SIZE to fgets
define the file name in a constant or a #define and use the file name in the
error message on failure to open.  The message should be produced with
perror

const char *dict = "wordDic.dic";
...
FILE *file = fopen(dict,"a+");
if (file == NULL) {
    perror(dict);
    exit(EXIT_FAILURE);
}


notice that I exited immediately on failure (your code continues).
perror will produce a short error message on stderr that says why the file
could not be opened, not just that it couldn't.  Also note that I checked
the file pointer immediately, not after prompting the user, as you do.
you ask the user whether she would like to perform another search
(on success) but then ignore the answer.
the call to fflush is strictly unnecessary.  Buffered output like
fprintf (and printf) writes to file when it sees a \n in the buffer.  In
this case, your strings have \n embedded (from the fgets call) and so
fprintf will flush the buffer; the buffer is empty when fflush is
called.  The normal thing would be for the word in wordContainer to have
no \n and for fprintf to use a format containing \n - fprintf(filePtr,
"%s\n", word); - and again, no fflush is needed.
your 

else
{
    printf("thank you for using this program,bye");
    exit(0);
}


exits before the final closing of the file and zero return.   Closing the file is good practice, although it will happen when the
program exits anyway.
I don't really like your variable names - three variables starting with
'word' is confusing.  I'd go for something like word for the user-entered
word, entry for the dictionary entry read from file and found to
indicate that the word was found in the dictionary.  That is a rather
personal preference though...
remember to put a \n on the end of lines that are printed, eg on exiting the
program.