You might have reached your exchange store limit. If it's over 16GB and can add a registry entry to increase it to 17gb. You'll then be able to mount the store.
1. Click Start, click Run, and then type regedt32.exe.
2. Locate the following key in the registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Exchange Server Name>\Private-<long hexadecimal string>
3. On the Edit menu, click Add Value, and then type the following in the Value name box: Temporary DB Size Limit Extension
4. Select REG_DWORD for the data type, and then click OK.
5. Enter a value of 1, and then click OK.
6. Quit Registry Editor.
After that's done, add the following entry to increase your store limit to whatever, because if it reaches the 17gb limit you will not be able to mount it again.
configure a logical database size (logical size means the physical size of the .EDB and .STM files minus the logical free space aka white space in each) limit for each Exchange database by creating a DWORD registry key named “Database Size Limit in GB”. This key should be created under the following location for the mailbox database and public folder database respectively:
* HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Private-GUID
Another reason might be that the server have not enough disk space