Sounds like water valve is broken.
You can go to toilet mfg. website and get manual with exploded view of parts.
Usually due to confined space/access it's easier to pull toilet off floor flange, take outside and repair/replace parts.
Which toilet do you have?
Sealand, thetford etc. and model
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31