To access the BIOS start tapping F2 immediately at boot. Run Diagnostics first though, tapping F12 immediately at boot, and watch for any drive errors as the power outage may have damaged the HDD/SSD.
My Latitude E747 laptop won't start after the power was cut to our house. When I press the power on button, the Dell logo appears and then a black screen. No BIOS info is displayed so I can't access the BIOS.  After several minutes a "grub rescue" prompt appears. (The laptop has LUbuntu installed which uses GRUB). The message GRUB displays is "error: disk 'hd0' not found". I think the motherboard, disk and powersupply must be OK because it can partially start GRUB (but I know nothing about this stuff, so that may be wrong). I tried removing the battery and booting with AC power only as suggested here , but it didn't help. I also tried powering off with F2 held down, and powering on with Esc held down (in case "fast boot" was enabled, as suggested here), but it didn't help.
Latitude E7470 won't start (Dell logo appears, but no BIOS info)