Yes there is a function. It is on the ribbon at Maths | Operators And Symbols | Operators | Calculus.
The summation symbol Terry pointed you to only works with column and row vectors (matrices with either just one column or just one row).
In case you are looking for the sum of all elements of an arbitrary matrix, here are two shorter routines which do the job:
And here is a way to sum up recursively all scalars even in multiple nested matrices (for whatever that may be worth).
Worksheet in format P5 attached
OK, here sure are many ways to skin a cat.
Even though DJV doesn't show any interest, here are two more ways to achieve the desired result.
Personally I still would prefer my first one, "SumAll1". It also seems to be the fastest, followed by the recursive "SumAll3".
Worksheet in Format P5 attached.