Generally speaking, you have to remove the old adhesive, clean off the residue with acetone/IPA and then apply new adhesive if you want the iPad to properly sealed. However, depending on how the screen came off and how damaged the old adhesive is, you could try to put the screen back with the old adhesive. The risk is that the screen won't lay flat or will lift in certain areas. FWIW, I would just order an adhesive kit. Just make sure to test everything before sealing it tight.