C# basic dictionary (hash) implementation
There are several (major) problems with this class:


Keys that contain a : character cannot be used to look up their value, and values that contain a : character are only partially returned. Also, this class cannot handle null values (it returns an empty string instead). Don't mash keys and values together into a single value, using a key-value object instead (such as KeyValuePair&lt;TKey, TValue&gt;).
Keys that start with anything but [a-zA-Z] cause an IndexOutOfRangeException. This is not documented anywhere, and there's no obvious reason why such keys shouldn't be allowed (phone books aren't limited to countries with a Latin alphabet, after all). Why not use the modulus of the hash value and the number of buckets?
Non-existing keys produce a 'magic' value, which isn't documented anywhere. Moreover, a caller won't be able to distinguish between a non-existing key and an actual (valid) "Not found!" value. You may want to throw an exception instead. Alternately, you could provide a TryGetValue method instead.
Overwriting the value for an existing key is not possible. Your class will store both values (why?), but lookups will continue to return the first value. I would expect the new entry to replace the earlier one. If that's not the intended behavior, then I'd expect that to be documented, and an appropriate exception to be thrown.
Your hash function is poorly chosen. Ideally, you'll want to spread entries across buckets as evenly as possible, so each lookup will take roughly the same amount of time on average. If items are concentrated in only a few buckets (some characters are more frequently used than others) then lookups for such keys will be slower. Why not use key.GetHashCode() instead, or let the caller provide a hashing method in the constructor?
Using a fixed number of buckets means that lookup times are essentially \$O(n)\$, not \$O(1)\$. Locating a bucket takes constant time, while searching through multiple entries in a bucket takes linear time, so you'll want to limit the number of entries per bucket (ideally to 1). That requires some kind of resizing strategy.


Other minor issues:


By calling Split(':') and HashFunction multiple times you're doing the same work repeatedly. Just call them once and store the result in a variable.
Some names could be clarified a bit: MyHashTable or MyHashMap instead of MyHash, and buckets instead of storage.
Going for a generic implementation would not only be of more practical use, it would also have helped you to avoid some of the above problems: you wouldn't have been able to mash keys and values together or to use a sub-optimal leading-character 'hash'.