Acer computer is frozen on

From the description you are giving, it appears that the system is not detecting a bootable hard disk drive. Hold the power button on the system till it powers off. once system is off, try the following:

1. Ensure no floppy diskette is in the A: if system has a floppy drive
2. Disconnect all external items (Printer, scanner, network connection) leave only video and keyboard connected to system.
3. Try powering system on again

If the system gives the same message, try rebooting and pressing F12 ONCE at the ACER screen and see if you can get a 1 time boot menu. If you can get the boot menu, select CD-ROM, and put you WINDOWS CD in the optical drive, see if system will boot to the CD. If you can, then at the first prompt, press R to enter recovery console, and see if you can run the command CHKDSK /R (then press enter)
If chkdsk runs, then reboot and see if the error has gone away. IF you cannot run CHKDSK then the hard disk drive has likely failed and will need replacement.