How can I find a Bitcoin private key that I saved in a text file? I certainly saved my private key in a text file previously, 
but I don't know where now.

Is there any way to recover my private key?
If you're programming savvy you could write a small program that searches for the regular expression of a Bitcoin private key. Bitcoin private keys have a very specific format:

They are 51 keys long, start with a '5' and the second letter is either 'H', 'J', or 'K' and use base58 for the remaining 49 letters which excludes the characters 0IOl.

It seems to me that the following regular expression should work to find an uncompressed private key:
5[HJK][1-9A-HJ-NP-Za-km-z]{49} (inspiration: Stackoverflow).

Later, another format was introduced for compressed keys, which has 52 base58 characters, and starts with a 'K' or 'L'. For that format, you should be able to use the following regular expression:

[KL][1-9A-HJ-NP-Za-km-z]{51}

Note that regular expressions can use slightly varying syntax, so you might need to adapt the expression to the programming language or tool that you use. You can generate a few private keys, e.g. with bitaddress.org for testing material. You should probably be more effective if you include word boundary \b symbols before and after the expression.

If you're on a Unix system, these grep commands would find uncompressed and compressed private keys recursively from the directory you're calling from:

grep -r '\b5[HJK][1-9A-HJ-NP-Za-km-z]\{49\}\b' *
grep -r '\b[KL][1-9A-HJ-NP-Za-km-z]\{51\}\b' *


Or search for both of them with this combined pattern:

grep -r '\b[5KL][1-9A-HJ-NP-Za-km-z]\{50,51\}\b' *


To use grep on Windows, you could perhaps find a solution here: What are good grep tools for Windows?