My boiler wont ignite.I replaced a 3a fuse & brought O2 pressure up.But no ignition.Its a combi.Please help!!!

The fuse I replaced blew on Sunday (for reasons unknown to me) but I didn't get to replace it until Wednesday. As I said the water pressure is fine, the fuse is too. But being gas I haven't tinkered around with it for fear of killing myself and everyone within 100 yards. Is there anything I can do to short of calling out istance only to find it's a simple solution and inadvertently announcing my stupidity to the world?