ALTER COLUMN NOT NULL, not enough space in the log file
you might find it faster to create a new table, truncate the old, and then re-insert:
select t.*
into temp_my_table
from [dbo].[table_name] t;

truncate table [dbo].[table_name];  -- back it up first!

alter table [dbo].[table_name] alter column [Value] [float] NOT NULL

insert into [dbo].[table_name]
select t.*
from temp_my_table t;

You may have to pay attention if you have insert triggers or identity columns.