No, you can't "undelete" a partition, but you can create a new partition in the space it occupied. Or you can use various programs, some free, some not, which will resize your existing partitions to utilize the free space.
If you use such a program, be sure to have a backup. While rare, it's possible for resizing of partitions to go titsup, and you lose all data.
I had three partitions on my hard drive. Using diskpart I deleted one partition not realizing it would also delete the space available on the drive. (Dumb) Is there a way to undelete this partition without having to buy software? I've tried a few demos but they only analyze. Thanks.