1840 nem coins disappeared

Hello,

 The software wallet  have just withdraw 1840 coins in my wallet. I dont know how it could have happened . i checked it once in 2 weeks or so and it is a private computer.  It happened on oct 9 and i just saw it now . Can someone help ?

How could this happen and how do i get it back ?
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This is the transaction detail:
Transfer transaction
From
NA2FHU-SIUI65-YQSRBJ-4D6ZWH-ZS7X2T-D5TSFO-R7WX
To
NBLQ6P-E7Z5CV-ANJNXG-OR74UQ-LOJ2YM-GJJOZ4-YFAQ
Amount
1 840.000000 XEM
Fee
0.150000 XEM
Message
5d19a020ad3211e7a6c717a42ec3df90
Block
1321097
Hash
7bbde0069d3f55e0e26744a05d8f261d50dbacc0417cbb6985241bb529372de8