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... Thom

 

You can flash official samsung stock firmware via Odin. You will have to download the requisite files including the odin program, the firmware as per your device model and the region plus any ops files if needed.
Visit sammobile.com and lookup the things needed.

Be very careful and follow the instructions, make sure to refer more than one site for the instructions, if this is not done right you might turn your phone into a paper weight.

You can also take it to the samsung service center for reinstallation of the firmware if you want.

 

I am an s5830 user but since your device is not very different so i think the procedure will not be very different from flashing ACE S5830.
In case you did not find firmware at sammobile you can try here->Samsung S5830 / B / C / D / G / I / L / M / T / V / Z / S5839I Galaxy Ace Firmwares
Firmware come in two formats, single package .tar format and multiple files like shown in the below guide(link below).

I found this guide for using Odin->Unbricking Samsung Galaxy Ace s5839i - xda-developers
You can try this and ask your questions here, they are all s5839i users and you will get better help there.

Refer this for using odin, its for s5830 but it should give you an idea what to do->http://androidforums.com/ace-all-th...rick-upgrade-downgrade-your-sga-gt-s5830.html
Just see how odin is being used here.

 

I already gave you 2 sites from where you can get the firmwares.

You don't necessarily need multiple file format to flash, you can use a single format firmware also, just tick the box that says 'one package' in the odin program. Although i might be wrong because the odin version you have to use might not have that option, so take a look if its possible for you. I used odin 4.38 and that had the option.
People usually suggest multiple file format because it has a much better chance of reviving a phone but otherwise you can quite easily use the single format one.
Try it, if it doesn't work then look for multiple format version.