"...my dell inspirion b120 was dropped..."
Dropping a laptop or netbook often damages the mboard or other components inside the case. That's a frequent reason something inside the case needs to be replaced.
Some laptops may not power up when only the AC adapter is connected and the main battery has been removed, however, if you're sure it was able to power up in that situation previously, something is definitely damaged.
You can have problems with the AC adapter powering the laptop even when you have not dropped the laptop.
See the info at the link I referred to in response 1.
You may find that when you wiggle the cord near the plug, or wiggle the plug, when it's plugged into the jack, the "charging" led on the laptop may go on and off when the battery is installed.
If you're willing to try figuring out what the problem is, search the web using: inspiron b120 manual - at least one of the "hits" will be on a Dell web site that has a Service Manual you can read online or download.
(I tried that with an Acer laptop - I found a free service manual for it's series on the web - it had been dropped - the corner of the mboard was obviously damaged - making sure all connections of wiring and cards inside the case were okay did not cure the problems. In that case the LCD display no longer had video immediately after it had been dropped and thereafter, and later on ALL the USB ports and the keyboard stopped working. At the very least, the mboard must be replaced.)
If you do that and can't figure out what is wrong, or otherwise, take it to a local place that is authorized to work on Dell laptops and have them analyze it and make an estimate of what it would cost to fix it.
Wther you choose to fix it yourself or you get them to fix it, you / they can often buy new or used mboards for Dell laptops on the web for a reasonable price (if yours is not a recent model, the price is often cheaper), but if you do that yourself you need to determine it's Dell part number - that's probably on a stuck on label on the surface of the mboard somewhere.