Genuine problems are fairly rare, but unfortunately there are a few "features" which can cause problems.

No beam 

Is the front end open? Is the safety shutter open? Should the monochromator feedback running? Is the undulator gap in the correct position, sometimes it is opened but not closed again after a refill...  

Software problems

If Bliss hangs try Ctrl+C, you can also close the session by using exit and relauching it. If motors are not responding after a Ctrl+C: try the sync() command. 

Safety Interlocks

All hutches must be properly searched and locked before the beam can be delivered. The EHO (ext. 2525) can usually assist if you have problems. Various programs are available from the unix machine id11 to assist with monitoring the state of the beamline. The idappli program gives details about the front end, jvacuum gives a graphical view of the safety shutters and vacuum system (valves must be open!) and finally jpss gives a logical diagram showing where an interlock might be.