What could be a possible reason for computers to re-boot

not enough ram memory
chip not fast enough
os system too old
discuss possible slow operations with a computer repair shop

So either the computer isn't generating a video signal, the cable isn't carrying the signal to the monitor, or else the monitor isn't capable of receiving a video signal.To test this, first try a video cable that you know is good, in case your cable is bad, If it works with the other cable, the problem was your original video cable. If it still doesn't work, put your original cable back, and hook another computer up to the monitor. If the monitor makes the same message, the problem is with the monitor. If the other computer works when connected to this monitor, your problem was that the original computer isn't booting (which could be due to several reasons I will list below), or the video adapter is broken.Possible reasons for the computer not booting: bad power supply, bad power button, bad hard drive, bad memory, bad motherboard component. If you determine that it isn't booting, unplug everything except the keyboard mouse monitor and power cord, to see if that makes a difference. If it boots after that, something you unplugged was causing the problem, plug them each back in one by one, rebooting in between, and when it stops booting properly, the last thing you plugged in is causing the problem.

initially Switch the computer on and keep pressing the F8 key until a repair screen shows up.
Start this in the safe mode and see if you get your screen back

I suspect that the hard drive motor is going out. Try a new hard drive. If it doesn't fix it, you can always return it.
Good luck,
James.

Check programs at startup menu. There should be some programs whis downs computer speed after boot

As you have written that your CD ROM is working fine, then there is only one possible reasons for this, that Your BIOS is malfunctioning and you have to download and install the latest version for your system.

will it boot into Safe Mode?

if it wont boot into safe mode, then its a pretty big problem. your laptop might now be garbage.

your going to have to bring it to a store that fixes computers...odds are your laptops video card is integrated into its motherboard...which means the fix is replacing the motherboard. (the smarter option would be to just replace the computer).
Another possible reason is that the hard drive is now junk...which means you (or the computer store) have to take it apart, ans swap its hard drive out with a new one.

either way...its no quick fix.......sorry.

Possible reasons:
1) short circuit between board and case. Check fastening and screws.
2) Poor contact to RAM and video card. Clean the pins by alcogol.

There is one possible reason for this issue

1. CMOS battery

Troubleshooting Steps
Open the computer case and find the battery on the computer motherboard, verify that it will be accessible and that it can be removed. Most computers today use a coin cell CMOS battery. Removed the battery and put it back. This will reset the BIOS setting to default. Change the setting and select first boot device as CD ROM. If that doesn’t work then replace the CMOS battery. After replacing the CMOS battery please change the BIOS setting and select first boot device as CD-ROM.

Emachines have had a lot of problems with their power supplies going bad and destroying the motherboard in the process. If you can, start looking for a replacement motherboard (on eBay) and a larger wattage power supply (any local computer store) especially if you have experienced the computer shutting off or rebooting for no apparent reason. If you're lucky, you can try booting into safe mode by pressing the "F8" key on you keyboard during the boot process and select safe mode from the menu. If your computer boots up, then there exist the possibility of a corrupt video driver and/or corrupted file system. If you still can't boot and experience the same thing, then you may have a physical hdd problem or again, power supply, motherboard, or both. Also, do not over look the possibility of an internal fan on the video chip not turning. I hope this helps and good luck.