Try running Toshiba Factory Recovery now as it may still run if only C was overwritten. Toshiba HDD Recovery Utility
Be aware that you can simply insert a higher-version Product key in Anytime Upgrade to unlock the extra features in minutes.
If Toshiba Recovery does not run, post back a screenshot of your maximized DIsk Management drive map using Snipping Tool in Start Menu. There's a possibility if it still exists the Recovery partition can be made to run by marking it Active then rebooting: Partition - Mark as Active.
Note that you'd use Method Two to mark the current System Reserved (preferred) or 7 partition Active again if necessary. Take note of which is marked Active now.
If this fails, you'll need to order your Recovery Disks from Toshiba Tech Support, or find a Windows 7 retail installation DVD to clean reinstall Home Premium following these steps to get a perfect Reinstall: Reinstalling Windows 7
There is also a workaround to change between versions by changing the Registry key shown below to HOMEPREMIUM
for the EditionID and Windows 7 HOMEPREMIUM
for the ProductName., then run an in-place Upgrade with the Home Premium installer. Back up your files and set a System Restore point: