That depends on which valve and what is wrong with it.I presume it is the flushing valve. If it is not flushing, the arm may have broken inside the cistern. If it is continually flushing, the seal is probably gone inside the valve.
You are probably better to get a new internal unit ~£15 from screwfix. Or, if I have not given you the answer, let us know, and we will help.
If the button works but the light doesn't, it will need a new LED on the board. That is not an easy job. There is a slight workaround though.
1. Go to settings
2. Ease of access
3. Keyboard (left pane)
4. Turn on USE TOGGLE KEYS
5. Audio (left pane)
6. Pick the option that suits you best on SHOW AUDIO ALERTS VISUALLY
Or get an external keyboard