I need to know why my gas boiler doesn't light. The pilot is lit but it will notstart?

Unfortunately, there could be several causes for this, and more detailed information and troubleshooting will be necessary to make a decision.

First, what makes a boiler come on? A thermostat sensor completes a circuit. Start there. A fuse or breaker could be tripped somewhere, or the system may have "Bad information" thinking heat is not necessary for some reason.
This almost will need someone on the sight to troubleshoot and make a determination, but maybe with some slow logical fact finding, folks here might be able to help!