Mountain bike front derailleur won't shift up

I didn’t use my bike for few months. Now the front derailleur won’t shift up.
Basically, when I shift to the second gear I hear a "click" then the chain is stuck between the first and the second gear. Futhermore, I can’t shift to the third gear I don’t hear the "click" and the chain is still stuck between the gears.

The derailleur cage seems fine to me. The cable is not too loose.

this is my first time I try to repair my bike myselft. I don’t know what else should I check.

Edit: The chain is not really between the first and the second gear. The chain is more like half on the first gear and half not touching any gear. It feels like the chain is not high enough to reach the second gear.

Edit: The derailleur should be 2mm above the outer chain gear, but mine seems to be 0.5mm above. I don’t have the tools today to test it. Maybe this is the problem?

update:While pedalling if force a bit more over the second gear using the shifter on handlebar I can get on the second gear then 2 to 3 works like a charm.