I'm in need of help, my Minecraft will not launch

I try and launch it, then the little green loading bar appears. Once it is done loading a Java error comes up called "Java Virtual Machine Launcher" with a box of text that reads, "Could not find the main class: net.minecraft.client.main.Main. Program will exit."
I'm guessing one of the files is missing but I don't want to mess around with things without knowing what I'm searching for. I have not played Minecraft in quite some time, but I know the client is up to date. Anyone got any solutions to this problem?

Check you have latest Java version.

First, update Java to the latest version. Second, what you want to do is go into your minecraft directory. If you do not know how to do this, go into your Start menu (if you have Windows it should be the res, green, blue and yellow squares) and search for "%appdata%", and there should be a folder called Roaming. Click it, and open ".minecraft". If you do not have this, reinstall minecraft. If you do, backup your mods before reinstalling, if you have this. BUT before you reinstall, if you have.minecraft, open minecraft and get that crash report again. Next, re-open.minecraft to reset the folder, then open "crash-reports" and it will show a list of documents showing all your crash reports, with dates. Get your most recent one, and then go to "dinnerbone. Com/ mojira/ crash_search/", removing any spaces. Paste your crash report, and if it has already been reported, there will probably be help there. If not, reinstall and hope for the best.