Originally Posted by MistahBungle
Download stock rom for country/carrier from Samfirmware, boot into download mode, flash with Odin following instructions in thread stickied near top of Q&A titled 'Flash stock firmware', problems with flash = find Hopper8's 'Odin troubleshooting' thread stickied in General, it has all the info you need to get a successful flash.
Once you've done that, re-root your phone & restore nandroid backup or re-flash custom rom/kernel & start again.
But frankly, you should already know this stuff if you're modding your phone. Not knowing how to recover when things go wrong (and they do for everyone sooner or later) potentially leaves you without a working phone for hours (at least), especially given S2 Q&A doesn't get as many posters or people answering other people's questions as it used to.
Have downloaded the stock ROM, tried flashing it but it got stuck on
<OSM> Enter CS for MD5..
<OSM> Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> I9100XWLST_I9100ODDLSE_I9100DDLS9_HOME.tar.md5 is valid.
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/009> Odin v.3 engine (ID:9)..
<ID:0/009> File analysis..
<ID:0/009> Get PIT for mapping..
<ID:0/009> Firmware update start..
<ID:0/009> NAND Write Start!!
No progress after this point.