Open the pump lid. While running, yes while the pump is running, pour a large bucket of water as fast as you can into the basket. If the pump takes the water as fast as you pour it in. Might ( not always ) means its a plumbing issue. If not then it might be a filter issue. If you have a DE or sand filter try priming it first in backwash. If the pump tries to, or fully primes up it's the filter and needs cleaned.
When I run into those problems out of the blue it normally is the filter and needs to be cleaned. You should just start by cleaning the filter because that's likely your problem.