A 16x dvd +R cannot be erased. There is a type of +R disk that can be erased, with the designation +RW, but their fastest speed is 8x and are more commonly found in 4x varieties.
A regular +R disk will have a bluish, or purple-blue color to the writing side of the disk. An RW disk will look grey or black. This is because a regular dvd uses an organic dye layer that can only be written to once, and RW disks use a phase change metal alloy which can be erased and written to again.
Now it is possible that your burning program didn't finalize the dvd. In that case, it might be possible to use the space left on the dvd that wasn't written to originally. If you burn to the unwritten space, it is called multisession, and you'll probably lose access to the original material, and you'll never get back the space that was used in the first burn. I do not recommend multisession burns on dvds.
You'll be much better off getting some new dvds. Verbatim +R disks with AZO on the label would be a good choice. Don't get the "Life Series" Verbatim disks.