Remi wrote:
We just moved into a 60 year old house and discovered that my daughter's
electric guitar and amplifier won't work properly. There'a a loud hum
coming from the amplifier. It stops when she puts her hands over the
strings, but starts up as soon as she removes her hands. Could this be a
grounding problem? Any suggestions? (There's nothing wrong with the guitar
or the amplifier. She also has a bass guitar and a bass amplifier and the
same problem is occuring with them too). Thanks.

As mentioned, this is a grounding problem.
It could also be dangerous, since if there's a fault in the amp the
guitar strings/various metal bits could become energized, which when
the ground works properly will cause the breaker to trip.
No ground, no tripping breaker!

Until you are sure everything is OK, it'd be cheap insurance to add a
GFCI outlet or breaker, or get one of those extension cords with a
built-in GFCI.

Dave