Trying to vertically center text text is a common issue. Normally this wouldn't work on normal boxes, but you can force it to work with vertical align:
vertical-align: middle;
display: table-cell;
However this will not work in IE7 and lower.
If you are sure the text you want to display you could use line-height to fake the effect like this:
height: 40px;
line-height: 40px; /* same as height */
This way works cross browser and has support up to IE5.5 I believe. If this is not an option I'm afraid you're out of luck (it can't be done).
As a side note that error message suffers from bad grammar, it should be "Congratulations! You have successfully done it.".
Make it easier ;) ! Try to use alert-block ! That works fine with latest Bootstrap version !
<div class="alert alert-block alert-success" id="UploadSuccess">
<a class="close" data-dismiss="alert">×</a>
<strong>Congrats!</strong> You have successfully did it!.
</div>
Make the line-height
of the div the same as the height
of the div, eg:
#UploadSuccess { height: 20px; line-height: 20px; }
This will only work if you have one line of text.