The ! turns up when iPhoto loses the connection between the thumbnail in the iPhoto Window and the file it represents.
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Rebuild iPhoto Library Database from automatic backup.
If that fails:
Download iPhoto Library Manager and use its rebuild function. This will create a new library based on data in the albumdata.xml file. Not everything will be brought over - no slideshows, books or calendars, for instance - but it should get all your albums and keywords back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one. .
OS X needs about 10 gigs of hard drive space for normal OS operations - things like virtual memory, temporary files and so on.
Without this space your Mac will slow down as the OS hunts for space on the disk, files will be fragmented, also slowing things down, apps will crash and the risk of data corruption - that is damage to your files, photos, music - increases exponentially.
Your first priority is to make more space on that HD. Nothing else can be done until you do. Purchase an external HD and move your Photos and Music to it. Both iPhoto and iTunes can run perfectly well with the Library on an external disk.
Start over from scratch with new library
Start over with a new library and import the Originals (iPhoto 09 and earlier) or the Masters (iPhoto 11) folder from your original library as follows:
1. Open the library package like this.
2. Launch iPhoto with the Option key held down and, when asked, select the option to create a new library.
3. Drag the Originals (iPhoto 09 and earlier) or the Masters (iPhoto 11) folder from the open iPhoto Library package into the open iPhoto window.
Click to view full size
Check the new library to see if it includes all of your photos including the missing landscape photos from your original library.
This will create a new library with the same Events (but not necessarily the same Event names) as the original library but will not keep the metadata, albums, books slideshows and other projects.
Note: your current library will be left untouched for further attempts at a fix if so desired.