No, you cannot go back. Count how many different hashes you can have. Now count how many different strings you can have. The first is finite, the second is infinite. There are lots of (infinitely many, to be precise) strings which have the same SHA1 sum. The point is, however, it's very hard to find two texts, which have the same hash.
You can think of hashing as shortening something. For example take a hashing function which sums all the ASCII codes of the letters in a string. You can't tell what was before hashing, just knowing the sum of ASCII codes of the letters. It is similar with SHA1, but more complicated.
The point of hashing is not to encrypt something. The point of hashing is to shorten something, so that checking whether two things are the same takes less time. Now how can you tell that two things are indeed the same if you know that lots of things have the same hash? Well, you can't. You just assume that it's so rare that it won't happen.
But hashing is not just about checking, as checking equality using hashes is usually used just for confirmation/validation and it is not deterministic. If you see that hashes are the same, then basing on the parameters of a particular hashing function, you can estimate the probability that the hashed objects are indeed the same.
And that's why the fact that a hashing function always yields the same results for the same objects is the most important feature of a hashing function. It lets you validate and compare objects.
