The bash prompt is controlled by the PS1 variable in one of several bash config scripts. Here's a good article discussing how to go about changing the prompt to your liking:
This variable is probably set in /etc/bashrc rather than ~.bashrc. The former sets the bash configuration for all users globally while the latter makes changes only for the individual user. It's unlikely that either of these files was spontaneously edited so unless you have been fooling around with these files, my guess is that something changed in your /etc/hosts file. That's where the "computer name" is picked up. Check this file; it should minimally look like:
Instead of "local host" your computer name should be listed for 127.0.0.1. If there are other computers on your network, they may also be listed here with there IP address and name but you should always have something there for 127.0.0.1 which is your local machine. My best guess is that this file was modified in some way and your bash configuration is falling back to a default prompt because it cannot pick up your computer name from /etc/hosts.