There's quite a lot wrong with your statement. Here's a brief analysis:

declare @string nvarchar(MAX)

declare @UserMidListTable Table (Id Int,Mid int,ValueMid int,CatParent int, [Enabled] int,LastUpdate Datetime,Company nvarchar(max))

insert into @UserMidListTable (Id, Mid, ValueMid, CatParent, [Enabled], LastUpdate, Company)

exec UserMidList '5C34A797-DCCB-40FF-B7AD-F6512803065E','20','0','0','10','1','2','','' -- add by kashani

SELECT * INTO #temp1 FROM @UserMidListTable

SET @string=''

SELECT TOP 50

@string =

ISNULL(

'<tr> <td><a class="class_a" href="'

+ ISNULL(exhibitor.dbo.prpReadTypeID(d.Mid,d.ID,1,90),'HTTP://namayeshgah.com')

+ '" target=_self> <div id="class_ans"> '

+ ISNULL(exhibitor.dbo.prpReadTypeID(d.Mid,d.ID,1, 150),'') -- WHERE DOES [Mid] COME FROM??

+ ' </div> </a></td></tr>'

,'')

+ @string

COLLATE DATABASE_DEFAULT

FROM (

SELECT top 100 percent -- WHY?

m.ID,

ROW_NUMBER() OVER (ORDER BY m.ID) AS [Row] -- WHERE IS THIS USED?

FROM exhibitor.dbo.maintable m

INNER JOIN #temp1 t

ON m.ID = t.ValueMid

INNER JOIN exhibitor.dbo.blgbelongs b

ON b.Table1ID = t.ValueMid

WHERE b.table1ID IN

(SELECT bi.table1ID FROM exhibitor.dbo.Blgbelongs bi WHERE bi.Table1 = 20 AND bi.Table2 = 12)

) d -- MISSING IN ORIGINAL CODE