Site keeps going down - Error '500'

When I try and access my web site, I keep getting a failure page, that says:
"The page isn’t working is currently unable to handle this request.
I contacted my web host who said it was working fine at their end and for me to clear my cache etc. I tried that and am also checking from another location, but same problem.
Can you shed any light on where this error is occurring please?
thanks, Neil.