Just as plaintuts explained, you will need to reboot the computer using the startup recovery disk. The acer recovery disk set came with the computer when you purchased it. In some instances, the manufacturer does not include it with the purchase of the computer. You will need to purchase the disc from the manufacturer or from a third party. If the manufacturer doesn't find it, look for your disc at
Recovery-Disks.com We use third parties to get recovery disks that the manufacturers dont have, or if customers need them quickly. If you cannot find it on the site there, call them to see if they carry the disc.
Once you have the disc, follow the onscreen instructions to install them. This will reinstall the operating system and the drivers that your computer needs to function correctly. Once you complete the process your machine will be up and running again. Post here if you have any questions or problems with the installation.