A cost effective repair may well be possible if you are good at DIY.
Initially I would try a reset, press and hold furthest right key, then turn on. This is pretty standard for most Yamaha keyboards.
If you still have an issue then open up the keyboard and go though all the connections to ensure good contact. Look for discoloured electronic components and take note of any burnt plastic aroma that may be lurking.
An online search will reveal a service manual which should explain how to access the internals.
If on the other hand you are not good at DIY then a potentially aged keyboard is unlikely to be cost effective to repair if you have to take it to a Technician.
A new Yamaha PSR SX700 is likely to be c£900 and a similar price for a Korg Pa700 should you consider getting a new keyboard.