I need to reinstall ubuntu

If doing outside OS like on startup .. It has to be Bootable (LIVE) usb ... Also Bios has to be set to boot from USB device .. assuming you have the proper ISO file on USB .....http://www.pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/ .. will show you how too do such ..

Did you install Ubuntu using Wubi or did you boot the liveCD and install from there, resizing yourNTFS (if you dual booted)?

did you change the BIOS setting to boot from external-device ....??

I installed it using wubi, and no I havent changed the BIOS setting. I just replaced the windows 7 for the ubuntu but it showed some problems and the windows 7 is gone

Please explain more clearly what you are trying to do: you say that Windows 7 is gone. But you mentioned Wubi (which is installed inside Windows)

So are you trying to install wubi? or trying to install Ubuntu from a USB?

Thanks

I used to have Windows 7 and installed Ubuntu using wubi. That way I replaced Windows for Ubuntu. Now im having problems with it, and i think something went wrong during the installation, so i want to reinstall it using a USB flash

That's pretty much it

Wubi doesn't replace Windows. Wubi is an ubuntu installer that runs inside windows, and it allows you to dual boot Ubuntu with Windows using a virtual disk.

If you want to replace Windows you need to boot from an Ubuntu USB or Ubuntu CD - not run it within Windows. So if Windows is gone, I assume you are trying to boot from the USB. In that case you need to set your boot order (in the BIOS settings) to boot from USB (removable devices) before the main hard drive, or better, press the BIOS boot options key on startup and select to boot from the USB.

I know what you want to do: "install Ubuntu with a USB", but I don't know how you are going about it, at which point it is getting stuck and any messages you get. More info will help diagnose the problem. Start by how you created the USB stick.
Thanks