Most likely you were working with the older (version 7.0) IR set before and you recently only installed the Altiverb 7.2 updater. This new Altiverb 7 however requires the new IR set (available since version 7.1).
To fix this I recommend you to download the full (5 GB) Altiverb 7.2.7 installer and run this.
Once you installed this you will have all IRs again.
You can request a link to the latest Altiverb 7 downloads page at: