Stay calm, apple support is excellent.
Do you keep it up-to-date when new versions of iOS come out? If not, try that first, plug in to iTunes and synchronise - it should prompt you if you are due an update. Failing that...
Have you tried a reset (hold down the power and home buttons together, even after it goes off, until the apple appears, then let go)?
If that doesn't work, hook it up to iTunes, take a backup, disconnect it, do a reset (settings menu on the phone) and restore it from iTunes.
Failing that, go visit an apple store if there's one nearby. If not, give them a bell, they're agents are a bit OTT but helpful - good luck.
What model is it and how long you had it?