No straightforward answer to this, I feel.
When you say 'change', what do you mean? A completely new PC? With software pre-installed? In which case it should be working, so go back to the supplier and complain.
An upgrade someone did for you? And helpfully copied all your old stuff onto the new one? If so, they didn't do a proper job, complain to them and get them to do it right.
To answer the questions: on a new operating system it's done largely as a matter of personal preference; it could have bene just a C: or a C: & and F: or a C:, F: & G: etc. Normal convention would have been to just have a C:, though. Each one of these 'letters' is called a logical drive, and your disk may have one or more partitions containing these logical drives.
Simple to fix: no, complexity could range from straightforward, if tedious, to a right complicated hassle. You would (probably) need a 3rd party program, like Partition Magic in order to achieve it (if you don't have another PC, network or disk drive and all the disks to reinstall your software).
My advice is let whoever did it put it right.