FileVault decryption accidentially started - what will happen?

FileVault decryption does not erase any data or reset your computer.

When the decryption process is finished, the computer will be just like before - except that the data on disk is no longer encrypted. You can use the computer while it is decryption.

As this was done by accident, it is a good idea to enable FileVault again when it is done decrypting. It will unfortunately take a while to encrypt again, but you can use the machine while it is encryption.

There's no way of stopping the current decryption process to immediately encrypt again. You'll have to wait for it to be fully decrypted, and then enable FileVault again.