Inspiron5150 don't come on and I have a blank screen and underscore....

How do underscore on screen keyboard

Since when you are facing this problem, did you make any recent hardware or software changes ?
And after you turn on your laptop, don't you see anything after that cursor(Underscore) blinking ?
I mean it boots to windows or not ?

If not try to remove your battery and keep it aside for 5 Mins. and then connect it again. Then try to turn on the laptop

try un plugging the power cord and battery from the netbook , then press the power button for 5secs. then place back the battery and try to power it on with the power cord connected.

if you can get into the BIOS-setup, verify if your hard drive is recognised as a bootable device and set as first boot device. if the disk is not recognised, you might want to verify if the P-ATA/S-ATA controllers are enabled. if this is all the case, but the problem remains, then your hard disk is probably broken.

There might be one step you could try to restore the system to factory specifications. When you turn the laptop on, and you see the Acer splash screen, press the Alt-F10 keys together at the same time, and this should hopefully bring the Acer eRecovery system up, provided that your computer has the built-in recovery partition. If it asks for a password, try using the default of "000000". If that doesn't work, then a password has been entered into the system, and there is no way to reset it, however there is a way to find out what the password is, however it does require the use of the Ubuntu Operating System (which is free to download, here)

This new system I'm using has done something similar on a few occasions. I only had to disconnect/reconnect the plug from the back of my monitor and did the same with the Vdo card plug and everything was fine. That hasn't happened again since I put a different power cord on it.

When ready to hit enter the email address, in either edit or new entry mode, tap the top left button where it indicates "Abc". You will see a screen where choice five is "Symbols". On the first screen of "Symbols" the underscore does not appear. Right arrow your toggle button to the second screen of choices and then toggle down and over to the underscore, then tap, ok. - trevsmom

ok...do you have the install CD around? Slip that into the cd/dvd reader, and restart. The system should read that, and ask to reinstall your o/s. Hopefully, your data is still good on your hard drive.

Press and hold the # key until the symbols screen appears. Then use the directional keys to move to the set that contains the underscore, hit the number key that corresponds with the underscore and hit OK.

Is your HDD LED flashing? -Try pressing F8 on startup to show the boot menu option of your OS if it is Windows. If it does appear use Safe Mode first. If not successful try re-installing your OS.