I'm no longer able to post ads on Craigslist

You may have been banned for bad content?
Check you have not had identity theft or a virus / malware.
Use your installed antivirus software to scan, also consider free online scanning.
If you cannot find any reason then I suggest you create a new Craigslist account.

 Free Antivirus Editions:

Either of these three can be considered as the best choice.
(listed in order of my preference)

Panda:
http://www.cloudantivirus.com/en/forHome/

Bitdefender:
http://www.bitdefender.com/solutions/free.html

Avast:
http://www.avast.com/index

 
If you suspect malware.
Download these free cleaning tools.
Boot the PC in safe mode then run each of these free cleaning tools until the problem has beed removed:
 

1. Malwarebytes http://www.malwarebytes.org/

2. Combo Fix http://www.bleepingcomputer.com/download/search/?keyword=combofix

3. Rogue Killer http://www.bleepingcomputer.com/download/roguekiller/

4. Hitman Pro http://www.surfright.nl/en/hitmanpro/

5. TDS Killer http://www.bleepingcomputer.com/download/tdsskiller/

6. SuperAntiSpyware www.superantispyware.com

I tried posting it again and it says it posted but I search for it and it's not there. Any tips on how to go around that?

Contact Craigslist and ask them.

it's not malware or virus and I've tried contacting craigslist with no luck.

Keep trying.  It almost has to be a manual action against your account, so only someone from Craigslist will be able to resolve it.

You Have Possibly Been Ghosted by Craigslist

Ghosting is the practice that when your ads appear to have posted successfully, Craigslist actually hides these ads from users.  These ads are not searchable nor do they appear on the site. Essentially, Craigslist is tricking you into believing your ads are posted when they are not. These hidden ads, we refer to them as being “ghosted”.

http://weblanwan.com/ghosting/

thanks I created a new account.

glad to help.