Sounds like the drivers possibly corrupted, best thing to do is delete the driver and re install it, simple process.
If you right click on 'My Computer' or 'Computer' depending upon your version of windows, then click on 'Manage'
This takes you to the Computer Management Control.
On the left hand side you will see 'Device Manager', you need to click on that option.
This then brings up all the hardware devices associated with the PC/Laptop as well as connected devices.
Now the PicoScope driver can be in either a Pico Technology Section, or the Universal Serial Bus Controllers' section.
If it is the driver corrupted, you should see an unknown device with a yellow marker on it, or you could unplug the scope and re-plug it in to identify the driver.
Once you've located the correct driver, simply right click on it and click on 'Uninstall'
It should then ask you to confirm and there will be a check box asking to delete driver software. Tick this and then click ok.
Once its uninstalled the driver, simply disconnect the scope, re-connect and it should re-install the driver.
Once you see a message saying 'PicoScope 4000 Series Oscilloscope' successfully installed, simply re-open the software and you should be good to go again.
If you get stuck let me know as i can email you some screenshots or if it doesnt work, then try Pico Support as it could be something different but i suspect its just a corrupted driver.
Lee - LMH Engineering Services