Osciloscope issue

CYH2018

Hello, i would like to ask for advice, as i have a problem with my osciloscope.
Osciloscope model: DSO4254C
Ive Been using it for a few months now and everything worked perfectly. Today when i was doing measurements i was not able to use Channel 4. It was enabled but there was no green line as it should be. After trying to figure out what happend i noticed that the line is actually High up but when i tried to bring it lower with a knob it kept going up instantly and i was not able to see it. It behaved like the voltage at the probe is infinite, even tho it was shorted to its ground. Unplugging the probe didnt help. Other 3 Channels work as they should. I tought messed something up in settings and i recalled default settings. It didnt help either. I tried restarting and such, went trough all the settings but everything was correct. I could still use the 4th Channels knobs to change the voltage range or move the waveform around but it still was be having like if there was infinite DC voltage. Only way i could bring it to ground was setting coupling to ground but then i wasnt able to do measurements either. I could aslo single and force trigger to see the line if i moved the waveform but once i resumed it by RUN button it kept going up So i couldnt see it. I am using it every single day and i really wish i could fix it at home without sending it anywhere unless it is the only way. Overall it is a very great tool for electronics lab. I think this is all the usefull information i can provide.

Thank You for Your time and support. I cant explain how gratefull i will be for any advice.