So I stupidly tried to jailbreak 4.3.1, which I didn't know it wasn't the best idea. After getting my phone stuck in some kinda mode, I restored it, but iTunes forced me to restore and upgrade to 4.3.1. So my phone is basically useless at the moment, but I really need it in case of emergency. My phone is not unlocked or jailbroken. I didnt shsh my phone either so I cant downgrade. Is it possible to buy a prepaid sim card from AT&T and use it in my phone? Sorry if this has been asked a lot. I'm still learning.

Without saved SHSH, you have very few options. I can't give you a solid answer because you haven't mentioned what baseband version the iPhone currently has.

There are failbreaks that can jailbreak/hacktivate 4.3.1 (3rd party pwnagetool bundles) but these are limited to tethered jailbreak.

Well, I've always had the 5.11.07 baseband, but I just restored and updated it. Does the update change anything? And does the baseband matter when it comes to a prepaid sim card?

Originally Posted by KrisEv

Well, I've always had the 5.11.07 baseband, but I just restored and updated it. Does the update change anything? And does the baseband matter when it comes to a prepaid sim card?

Restoring/updating (without using custom IPSW) WILL update the baseband to the version that is included in the IPSW.
If your phone is locked to AT&T and you don't currently have an AT&T card then i'm guessing you were relying on ultrasn0w for unlock (before you tried to restore/jailbreak 4.3.1)

Now your unlock options are much more limited (as of today, you are limited to the irreversible and not advisable 6.15 iPad baseband as your only unlock option)

The best result you can get from an AT&T prepaid sim is teh ability to use your iPhone like an iPod touch.

Originally Posted by Olethros

The best result you can get from an AT&T prepaid sim is teh ability to use your iPhone like an iPod touch.

Really? I thought it works because I've read elsewhere about some people who have been using their iPhone on a prepaid AT&T sim for years with text and even a data plan. Apparently they activated the sim using another phone's IMEI number and just plugged it into their iPhone. :/

Any suggestions on how I can use my phone again?

That kind of thing might work, but AT&T can detect it and they are getting more strict about requiring an iPhone data plan (and detecting people who try to avoid this requiremnt)