Lift the front of the bike and turn the wheel by hand. Then apply the brakes.
Do the calipers release the discs, or do they hold on?
When you release the lever, does it go back to its position or does it stay loose?
When were the brakes bled last?
Anyway, bit hard to diagnose over the internet, but most likely the piston(s) in the calipers are sticking.
Calipers may need a rebuild. Plenty of videos on youtube showing how to do it, if you are mechanical.
The seals inside the caliper need to be installed correctly and carefully, otherwise brakes don't work and
you won't stop. If you are not mechanical, then perhaps better to take it to experts.
