More information needed for someone to help http://forums.adobe.com/thread/416679
Some specific information that is needed...
Build a Configuration file http://www.piriform.com/speccy
Brand/Model Computer (or Brand/Model Motherboard if self-built)
How much system memory you have installed, such as 2Gig or ???
Operating System version, such as Win7 64bit Pro... or whatevevr
-including your security settings, such as are YOU the Administrator
-and have you tried to RIGHT click the program Icon and then select
-the Run as Administrator option (for Windows, not sure about Mac)
Your Firewall settings and brand of anti-virus are you running
Brand/Model graphics card, sush as ATI "xxxx" or nVidia "xxxx"
-or the brand/model graphics chip if on the motherboard
-and the exact driver version for the above graphics card/chip
-and how much video memory you have on your graphics card
Brand/Model sound card, or sound "chip" name on Motherboard
-and the exact driver version for the above sound card/chip
Size(s) and configuration of your hard drive(s)... example below
-and how much FREE space is available on each drive (in Windows
-you RIGHT click the drive letter while using Windows Explorer
-and then select the Properties option to see used/free space)
Windows Indexing is BAD http://forums.adobe.com/thread/676303
While in Properties, be sure you have drive indexing set OFF
-for the drive, and for all directories, to improve performance
Some/Much of the above are available by going to the Windows
Control Panel and then the Hardware option (Win7 option name)
OR Control Panel--System--Hardware Tab--Device Manager for WinXP
And the EXACT type and size of file that is causing you problems
-for pictures, that includes the camera used and the pixel dimensions
Plus Video-Specific Information http://forums.adobe.com/thread/459220?tstart=0
Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037?tstart=0
What is a CODEC... a Primer http://forums.adobe.com/thread/546811?tstart=0
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037?tstart=0
Report back with the codec details of your file, use the programs below
For PC http://www.headbands.com/gspot/ or http://mediainfo.sourceforge.net/en
For Mac http://mediainfo.massanti.com/
Once you know exactly what it is you are editing, report back with that information... and your project setting, and if there is a red line
above the video in the timeline, which indicates a mismatch between video and project
Also, a common reason for a project that suddenly stops working is that you've suddenly run out of workspace.
Premiere Elements needs you to have at least 20-30 gigs of free, defragmented space on your C drive.
And, if you are editing to a second or external drive, not only must it have space but it must be formatted NTFS.
Is that the case in your situation?
Also, if there are red lines above the clips on the timeline, are you rendering them (by press Enter) as you work so that they turn green?