Beginner code for MadLibs game
You could use a bit less comments, most of the code is self-explnatory
You could also use a bit more split of the code into functions, e.g. a function to create a default file
This is a full script, so it's nice to have the if __name__ == "__main__": condition to call the main function
If you use the """ to have a multi-line string, whatever you put will be there, including newlines and tabs. You probably want to split the string by using " on separate lines (see later).
Also, if you concatenate that way, you're losing the \n, it's better to use format


Revised code:

#madlibs.py : ABS Chapter 8 Project
#Make mad libs using madlibtemplate.txt file that will prompt user for parts of speech
#Will return new file with replacements called madlib.txt

import re, os

def generate_default_template():
    with open('madlibtemplate.txt', 'w+') as template:
        template.write("The ADJECTIVE girl walked to the NOUN and then VERB. "
                        "A nearby NOUN was upset by this.")

def capitalize_sentences(text):
    sentences = re.findall('[^.!?]+[.!?](?:\s|\Z)', text)
    capitalized_sentences = [sentence.capitalize() for sentence in sentences]
    return ''.join(capitalized_sentences)


def main():
    print("Let's play MadLibs!\nNote: Your current working directory is {0}".format(os.getcwd()))
    if not os.path.exists('madlibtemplate.txt'):
        print("No template file found. Using default 'madlibtemplate.txt'")
        generate_default_template()
    else:
        print("Using your 'madlibtemplate.txt' file:")

    with open('madlibtemplate.txt', 'r') as template:
        template.seek(0)
        text = (template.read()).lower()

    wordregex = re.compile(r'\w+|\s|\S')
    words = re.findall(wordregex, text)

    for i, word in enumerate(words):
        if word in {'adjective', 'noun', 'adverb', 'verb', 'interjection'}:
            print('Enter a(n) %s ' % word)
            words[i] = input()

    madlib_text_unformatted = ''.join(words)
    madlib_text = capitalize_sentences(madlib_text_unformatted)

    print('Here we go... \n')
    print(madlib_text)

    with open('madlib.txt', 'w') as madlib:
        madlib.write(madlib_text)

    print("\nThis has been saved to file 'madlib.txt'")

if __name__ == "__main__":
    main()



If you want to capitalize a word, there's the... well, capitalize function for that.
When opening files it's nice to use the with keyword, so they'll be closed automatically.
You don't need to do a loop when checking the parts of speech, just use the in operator.