ok i will do that thanks for helping

Try getting the newest version of java, java run's pretty much everything, so updating your java, is your basic start, if that doesn't work, maybe try to download the game, instead of playing in browser?

good question :-p

Why would you even play Minecraft in a browser...

Like what?

won't i lose anything else??

Go to %appdata% then go to .minecraft then to saves.
Move all your saved worlds to your desktop or somewhere.
Delete .minecraft.
Download Minecraft again.
Go to %appdata%.
Go to saves.
Move your worlds you put on your desktop or wherever back in the saves file.
Run Minecraft and see if it works.

no if i delete .minecraft all of my worlds will be deleted! isn't there anotherr way to do it??

Or you can go to %appdata%
Then delete .minecraft
Then go to minecraft.net
login and wait until it downloads.
And if this doesn't work download the latest java

what client?

Open Minecraft
Click on options
Then Click on force update.
Then login.

i dont have any mods but how do i change to 64 bit?

You need to have java 64 bit installed if you have a 64 bit windows + like mrpipster said if you have mods you MUST delete META-INF

Oh, in browser, maybe java isn't running right. Click "run java plugin" or something like that.

Do you have any mods installed? Maybe you forgot to delete META-INF