I had this happen on a imported V3i back in the day so I know what went wrong. The software was lost to time and the phone needs to be reflashed. I bought an international model (unlocked) since the AT&T/Cingular version didn’t allow you to use MP3 ringtones.
That said, it may not make economic sense to get the phone relfashed depending on which version you have and the network limitations that you will probably have to deal with (International models have 3G bands, but NONE of them work in the US and the US phones flat out didn’t outside of models like the V3xx, which were typically exclusive). Outside of T-Mobile (and even that may end), nobody has 2G towers left in the US. I suspect the flash ROM on your phone may have also failed since it didn’t happen right away and only happened after 14 years of use.