The implant screw, which is in fact a healing abutment, is there for a reason. It prevents the soft tissue from collapsing into the platform head of the implant. If there is no screw and it happens, it will be difficult to connect the head of the implant to the implant abutment.
There is no need to be worried. Such a situation can be routinely solved at any dental implant clinic. Make sure you save the screw. The dental implant itself should stay in place without it. Find a local dental clinic which does implant dentistry. The dentist should be able to screw it back on. If you remember the dental implant brand, tell the dentist – it will useful information for them. It is highly recommended that the healing abutment stays on throughout the whole healing process. It is much better for your tissue and the healing process.