Types of Freezers
There are two main types of deep freezer: the upright and the chest. Many of these solutions work well for both styles. Removing the screws from the door, moving the door into the desired position and tightening the screws can help. Make sure the magnetic strip of your freezer door is clean as well. It can be cleaned with any mild dish washing soap and water. Dry the area thoroughly.
Door Position
Another possible reason your door won't close is its position. It might be sitting on a broken foot or have lost the correct pitch to close. Usually upright freezers are on a slight angle forward to help doors swing open and close.
Simple Solutions
If you own an upright freezer with double doors, a wrench or solid metal tool can often be wedged between the two handles to keep the door closed. For a single door upright, simply leaning a heavy object against the door (think a tire or workbench) will do the trick. For chest style, placing a heavy object on the lid should keep it closed. Several layers of duct tape can also be used for either style of freezer.
Door Replacement
If the door continues to be problem, you might need to completely replace it. Take down your freezer's manufacture information, including serial number and date of production (if known) and call your local home maintenance store to see if they have any available parts. If your freezer is an older model an auction house or individual seller may have just the parts you need.