It sounds like the sensor has started to fail or has outright failed for this device. Have you attempted to replace the batteries for the unit? Many times if the batteries are starting to give out the first thing that goes is the ability to sense weight.. it will still turn on but will not full function.
Please try switching to brand new batteries, if it still has the issue then the sensor is bad on this device. If you read around online you will see that many people have problems with these Homedics scales... they are giving many people trouble and just seem to give out all the time.
A much better scale would be something like this Omron here.. they last for many years in most cases, and do all the same functions as this Homedics (but much more accurately and reliably). You can see just based on the reviews it is a great device.. Nearly 500 positive reviews on Amazon really says something.. and for the price and free shipping you can't beat it:
http://www.amazon.com/gp/product/B000FJQ48I?ie=UTF8&tag=dependascient-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B000FJQ48IThanks, let me know if you have any questions or concerns, just reply to my answer here and I will be right back with you.
Tim