Most probably the plunger has unscrewed. You need to remove the plunger from inside. Remove the bottom circlip and remove that pin so that will be disconnected from the plunger mechanism. Remove the bolt and washer on top of the plunger, remove the group and than smartly tap with a rubber mallet to remove the plunger mechanism from below. You will probably notice that the shaft is unscrewed from the ceramic plunger though not all the way. Tighten this and clean inside the group. replace the mechanism from below and replace the sieve with the rubber o-ring seal - you will have to use a screwdriver or similar implement to secure the o-ring in the space. Replace the pin for the handle insert the circlip and replace washer and bolt. You are good to go.

That is most likely the issue & answer as described by rajuraja. For a quick, temporary fix—remove the lever pin that goes through the piston pin and then with a wrench, carefully turn the piston clockwise maybe 2 or 3 full turns. Be careful not to scratch the chrome on the grouphead. I turn and simultaneously lift at the same time. This keeps me going for several days, until the piston gradually unscrews itself. Put a couple of drops of PLA Turbo-Lock High Temperature Thread Lock on the threads of the piston if you decide to take the entire unit apart. Available at orphanespresso.com