We have run into this problem a few times, and see others that are still running into the issue. We have the EP43-UDSL Motherboard in two different systems, and it has happened to both at one time or another. Thought I'd post a few things that have fixed the issue for us.

Issue:
System with Gigabyte EP43-UDSL Motherboard will not post beyond the Gigabyte Screen, no access to keyboard and can not enter BIOS screen.
For us, it only seems to happen when the system has been turned off for a while.


Fixes:
Try Rebooting - Shut the system completely off, wait a bit and try booting again.
Sometimes will get it to go past the Gigabyte screen, but more often than not, you will need to resort to other things

Unplug Unnecessary USB devices - External Media card readers, flash drives, bluetooth dongles...there may be other devices that cause the problem, you may want to bring it down to basic Keyboard and Mouse.
After doing some extensive reading, I found that the BIOS for this board seems to have issues with some USB devices after the system has been shut down. On boot up, It gets stuck in a loop because it doesn't know what to do with a certain USB device. We tested USB devices one by one, and found that after unplugging an external media card reader, the system was able to boot in normally. We have also had it happen with a flash drive.

Reset CMOS - If the above still won't get the system to post, try resetting CMOS.
Just remember doing so will change any BIOS settings / overclocking you may have setup, so you will have to fix those again.

Hope that helps

~~ Gypsy