It used to not happen very often before, but now after I changed my motherboard it seems to happen way too much. Sometimes when I turn on my computer, the computer just beeps and the monitor doesn't turn on. If I push the reset button it does the same thing and nothing happens. The only way to GET it to boot is to push that power button in the back and then push power. What could be wrong?How old is this machine ? ?
Since you replaced the MBoard i'm assuming you are fairly familiar with the innards.
I would first replace the CMOS battery part#CR2032.
Make sure all power is dis-connected and you are properly grounded...
While you are in there check to make sure all components are firmly seated especially RAM and your vid card.
Also check all drive cables as well.
And do some housecleaning on the fans etc.
If it still does the same thing after this procedure then it sounds like your PSU is getting ready to take a dump.

Go here http://carpc.kyferez.com/Tools/PSUWattageCalculator.php for a HANDY PSU calculator to see what your machine needs wattage wise.
And do yourself a favor; don't buy a cheap one they are more headache than they are worth...

patio.  Umm well im familiar with almost everything inside my computer except the motherboard. Im sure the vid card is not loose I checked. Some other guys installed the motherboard for me.The CMOS battery is about the size of a dime on the MBoard. If you are not confident replacing it yourself call a local shop, they probably won't charge much to replace it for you.

patio.  Steven,

In addition to the other suggestions, try the following:

I've seen this problem before when RAM was GREATLY mismatched, failed or loose.  If you're not sure of the Ram, try installing one card at a time.  If the problem goes away, then add another.

Use ESD and electrical precautions when working around sensitive electronics.

Dell

Timesaver:  I always keep an older hard drive with the same system/software configuration I am using in my main boot drive.  This backup can sometimes be a real savior when repairing drive or system related failures.  Most of us have one or more 3-10 gig hard drives laying around from PAST upgrades.
I like this better than system restore when initially evaluating a problem since it doesn't depend on using a possible bad drive (with malware, viruses, etc.) to restore function.  You can use system restore once your problem is solved.