How do I reinstall RTL 120 BPL file?

The rtl120.bpl file has nothing to do with Microsoft Windows.

It is a Borland Precompiled Library file from a third party software vendor.

The rtl120.bpl file is from Embarcadero Technologies and gets installed on your system from third party software packages such as:

AVG  (their PC Analyzer component)

TuneUp Utilities

Advanced System Care  (ASC)

IOBit Malware Fighter Pro

For example, if you have Advanced System Care installed and the rtl120.bpl file that gets installed with it comes up missing, the message may look like this:

ASC.exe - Unable to Locate Component

This application has failed to start because rtl120.bpl was not found.  Re-installing the application may fix this problem.

There may be other third party applications that install the file, but if you don't provide the exact error message in it's entirety nobody is going to be able to help you.  And since the file has absolutely nothing to do with Microsoft Windows, maybe nobody in the Microsoft community will be able to help you without more information - like the complete error message and which if any of those third party applications (or those like it) you have installed.

If the system has been working okay and the file seems to now be missing for some reason, it was probably quarantined (effectively deleted) by your unspecified antivirus/antimalware software application(s).  Maybe you can find it as a quarantined file and unquarantine it.

With so little information provided, you can expect only theories and guesses about what the problem is and how to fix it.

If you have any of those kinds of third part applications, or applications where the product description includes such descriptive phrases as:

cleaner, mechanic, pit stop, detective, doctor, cure, optimize, inspector, clean up, tune up, speed up, fix up, compact, compress, faster, power, boost, booster, eraser, muncher. one click fix, instant fix, new, intuitive, mistakes, stops freezes, stops crashing, stops crashes, stops errors, improves, saves you money, risk free, guaranteed or any third party program that has an icon resembling a gear, cog, tool belt, wrench, spanner, screwdriver or swooping arrows?

You might have to uninstall and reinstall those applications (do what the error message says).

If you have that error and have any of those products installed, and reinstalling doesn't help, then you need to go to their support page for help since the file has nothing to do with Microsoft Windows.

You might be able to resolve the issue by simply uninstalling and reinstalling the afflicted product (do what the error message says).