What happens if I enable TPM on a running/already installed Windows OS?

By itself the TPM does nothing. It does not magically encrypt your disk, nor does it prevent your system booting.

The TPM is simply a secure enclave that provides security facilities.

It is where operating system programs such as Bitlocker can store keys.

If you have encrypted your disk then upon booting the (unencrypted) Bitlocker bootloader queries the key from the TPM and proceeds to transparently decrypt the disk. If the hard drive bitlocker booted from is removed from that PC and put in another then it will fail to find a key in the TPM, and will therefore be unable to decrypt the disk.

The TPM cannot do anything without your operating system or programs doing work with it. Just "enabling" the TPM will do absolutely nothing and will not by itself make files inaccessible.

Will I lose my files?

You absolutely will not lose your files. You don't have to even download drivers. Unless you use software that loads a key into your TPM, the functionality will simply be enabled, waiting to be used by the software.

The only software that I am aware of that even uses it on Windows would be BitLocker.

I wonder if I can be enabled now to be ready.

I enabled it on my personal machine without an issue.

Will Windows will keep booting?

It absolutely will keep booting.