So you have upgraded your computer and kept your existing external hard drive. Assuming that you have not reinitialised the hard drive (ie that you still have access to most of the other data that was stored on it), this could be a problem of the interface between your "old" driver and the new operating system — that, for no terribly good reason, your new OS will not read across the full range of disk addresses. (My printer stopped working when I upgraded to OS10.2.5.) Check with the manufacturer of the hard drive to see whether there are compatibility problems with your new OS. You may find there is a new driver on their website that will solve your problem. If that doesn't work, then you will have to get into the business of data recovery from the disk. Assuming that the files have not been corrupted, recovery should be possible.

PS. The change of location and of the ISP are not relevant.