To be perfectly honest there can be any number of reasons for loss of elasticity or sagging, ranging from premature ageing to severe sun damage to weight loss to genetics.
It would be difficult to answer you regarding your particular case since I have not had a look at your skin for myself. Thankfully regardless of the cause, there are a few things you can consider implementing as soon as possible. Firstly, to prevent and slow down the existing damage from getting worse, you need to use products that specifically deal with loss of elasticity, something like growth factors (Lamelle Dermaheal Or Skin Medica Recovery Complex). You need to protect the skin from becoming worse by preventing new and further damage, so the use of potent anti-oxidants is highly advisable (Skin Ceuticals CE Ferulic or Phloretin CF), and including a good facial SPF is equally important (Heliocare). Lastly you need to get that skin working optimally again, and for this purpose the inclusion of a retinoid night time product is a great choice (Neoretin Gel Serum Or Lamelle RA cream Or Dermaquest Retexturing serum).