If your refering to the gasket (rubber), stick a bit of silicone on the back of it and tap it back into the window. If you have difficulty getting it in, strech it by pulling it and then slip it back into place.A seal on my double glazing has come free internally. I can see it coming away. Ive read online it can be repaired for 40-50e rather than 300e for a new unit. Anyone know who can do it, meath/north dublin area.