No sound from my laptop speakers once I disconnected the headphones

my laptop is Studio XPS, and since morning I've had the same problem!

Have you tried contacting technical support?

Hey Reza!

I just found this solution over the net, it worked for me! it was amazing!

please have a read over it:

I think you have a problem with the headphones jack in your laptop. There are contacts inside the headphones jack, you can see them if you look inside the jack. When you plug the headphones into the jack, the plug shorts these contacts and the sound switches from the external speakers to the headphones. Apparently, these contacts got stuck in the headphones mode and because of that you cannot hear any sound from the external speakers.
I’m not sure if you can fix this problem yourself, probably you’ll have to take your laptop to the repair shop. Try plugging and unplugging your headphones a few times in a row, do it fast, it might help to release contacts inside the jack.

http://www.asklaptopfreak.com/laptop-notebook-help/2008/02/04/speakers-stopped-working-after-i-used-my-headphones/

"There are contacts inside the headphones jack, you can see them if you look inside the jack. When you plug the headphones into the jack, the plug shorts these contacts and the sound switches from the external speakers to the headphones. Apparently, these contacts got stuck in the headphones mode and because of that you cannot hear any sound from the external speakers."

Although that is true of many laptops computers and of headphone jacks in most electronic devices, in Dell computers with HD audio (that includes the Inspiron 6400 and the Studios) there is a different process going on. Those models have impedance sensing circuitry in the jack -- there is a sense pin in there -- that senses when a plug is inserted. The circuitry triggers switching software that turns the speakers on and off. When the pin fails then the software leaves the speakers off.  Sometimes inserting the plug and gently wiggling it will get the pin to work -- sometimes only momentarily, sometimes permanently, and sometimes not at all. There is no other fix for it in the 6400 except to replace the motherboard because the jack is not replaceable in that model.

Right click the volume button located at the bottom left corner, click playback device

Then, click Set Default.