Citroen C3 no longer starts

Hello all, I picked up a C3 – 1.4 ltr. – year 03, drove 100 km, started 5-6 times, everything perfect. Yesterday I set off the C3, tried to start it again later, the starter turns perfectly, but the engine does not start. I thought of the immobilizer, locked it up with the remote control and after a few minutes again, tried again, without success. The securing of the petrol pump is still tested, is ok.. I still believe in the Wegfa hrlock, since the starter rotates nicely evenly, but the engine does not make any effort to jump. Would it make sense to disconnect the battery for a few minutes or does someone have another tip for me? Thanks for every answer and greeting