You used the 2.71 downgrader?
Well now you're running the homebrew enabler do you have the official Sony 1.50 eboot on your MS renamed as UPDATE.PBP? (Sitting within "X:\PSP\GAME\UPDATE")
Go to the Photo menu, and select the HEN-D picture. If the screen flashes green you're good. Then go back to your XMB. Providing that you're in HEN, go to the game menu and select “1.50 Enabler" from the memory stick and run it. Then go to the Game menu and pick the "Update X.YZ" option from the memory stick. That will initiate your downgrade to 1.50.
From there install 3.52M33, update to 3.52M33-4 and then you're golden to get to the most recent firmwares. Readme's should be within all the M33 updates if you're uncertain how to upgrade your CFW properly.